I’m afraid the terms and conditions of the third-party maps generally don’t allow them to be downloaded in bulk, so no. You can view them but not download them for offline use.

Douglas, I think it's probably worth thinking about how useful the different map styles are on the app. On such a small screen the CT map is probably the best one, especially for navigation.

For planning, on the other hand, especially on a big screen, the extra maps can be very useful to gain different perspectives. The cycling maps, especially, can be quite useful. Even on the phone's screen (using the website) swapping between map styles can be beneficial. 

And, of course, those extra map styles are available to supporters, albeit online only. 

For me it's planning on the website (Chromebook or phone) and navigation on the app. Any planning on the app is a short term diversion. 

Hope this helps